Fuel assembly

ABSTRACT

A fuel assembly having a plurality of fuel rods arranged in a square lattice array. The fuel rods include a plurality of short-length fuel rods each having a fuel active length shorter than that of each of remaining ones of the fuel rods. At least one water rod is arranged in a region in which one or more of the fuel rods are arrangeable in the array, and a plurality of fuel spacers are provided at a plurality of positions in the axial direction for holding the plurality of fuel rods and the at least one water rod, with mutual radial intervals therebetween being kept immovable. The plurality of short-length fuel rods include at least one first short-length fuel rod arranged in the outermost peripheral region of the square lattice array. Each of the plurality of fuel spacers include a plurality of cylindrical members.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of U.S. application Ser. No. 09/245,899, filed Feb.8, 1999, now U.S. Pat. No. 6,400,788 the subject matter of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to a fuel assembly, and more particularlyto a fuel assembly, including fuel spacers, used for a boiling waterreactor.

A fuel assembly used for a boiling water reactor has been disclosed, forexample, in Japanese Patent Laid-open No. Hei 2-163695. This fuelassembly includes a plurality of fuel rods and two water rods. The fuelrods are arranged in a square lattice array of 9 rows×9 columns. Thesefuel rods and water rods, which form a fuel bundle, are held at mutualspecific intervals and are kept immovable by a plurality of fuel spacersarranged in the axial direction.

The fuel spacer includes a large number of cylindrical members; one bandmember; a plurality of loop-shaped springs; and water rod holdingmembers, each being formed into an approximately Ω-shape in transversecross-section, for holding the water rods. The large number ofcylindrical members are joined to each other and are bundled. Eachcylindrical member is provided at a lattice position at which a fuel rodis to be inserted, and an associated one of the fuel rods is inserted ineach cylindrical member. The band member, formed into a square shape,surrounds the outer periphery of the bundle of the large number ofcylindrical members.

The band member has projecting members (bath-tubs) which project on theinner peripheral side of a fuel channel box and is brought in contactwith the inner surface of the fuel channel box. The loop-shaped springis provided at a joined portion between a pair of the adjacentcylindrical members for pressingly supporting the fuel rods inserted inthe adjacent cylindrical members. In addition, the loop-shaped springfunctions to generate pressing forces when the fuel rods are inserted inthe adjacent cylindrical members.

The water rod holding member is joined to two of the cylindrical membersadjacent to the water rod. In the case where the loop-shaped springs areintended to be arranged for holding the fuel rods in the two cylindricalmembers adjacent to the water rod, since the mating one paired with eachof the two cylindrical members is not present from the viewpoint ofarrangement of springs over the fuel spacer, the spring function cannotbe achieved. To cope with such an inconvenience, a spring pressingprojecting piece and a spring holding projecting piece are provided at ajoined portion between the water rod holding member and each of theabove two cylindrical members, so that a pressing force necessary forholding the fuel rod is generated by holding the loop-shaped springusing the spring holding projecting piece and pressing it using thespring pressing projecting piece.

On the other hand, to improve nuclear characteristics of a fuel assemblyfor a boiling water reactor, as described in Japanese Patent Laid-openNo. Hei 5-232273, there is a known configuration provided with aplurality of fuel rods including rods (hereinafter, referred to asshort-length fuel rods) each having a length shorter than that of eachof the remaining ones of the fuel rods. In the fuel assembly disclosedin the above-mentioned document, to improve the controllability of thereactivity due to reduction in the void coefficient, the short-lengthfuel rods are arranged in the outer peripheral region of the squarelattice array and adjacent to the water rods.

The fuel spaces, as described above, are provided at a plurality ofpositions in the axial direction. As a result, of the fuel spacers usedfor the above fuel assembly, including the short-length fuel rods, thosepositioned above the upper ends of the short-length fuel rods have nofuel rod portions at lattice positions associated with the short-lengthfuel rods. In this regard, there has been already proposed a structurein which, in the fuel spacer positioned above the upper ends of theshort-length fuel rods, the cylindrical members located at the latticepositions associated with the short-length fuel rods are removed toreduce the pressure loss.

However, in the fuel assembly disclosed in Japanese Patent Laid-open No.Hei 5-232273 in which, the short-length fuel rods are arranged in theoutermost peripheral region of the square lattice array and adjacent tothe water rods, if it is intended to simply remove the cylindricalmembers located at the lattice positions associated with theshort-length fuel rods in the fuel spacer positioned above the upperends of the short-length fuel rods, the following two problems arelikely to occur:

(1) Problem in Terms of Strength

In a usual fuel spacer, short-length fuel rods are located in an innerregion (not outermost peripheral region) of a square lattice array offuel rods. As a result, even by removing cylindrical members located atthe lattice positions associated with the short-length fuel rods toreduce the pressure loss, the cylindrical members in the outermostregion are continuously in contact with a band member which surroundsthe outer periphery of the fuel spacer, so that the structural strengthof the entire fuel assembly is little reduced.

For example, if an external force is applied to the fuel spacer via afuel channel box in case of an earthquake or upon handling of the fuelassembly, the load is first transmitted to projecting members providedon the band member. After that, the load is transmitted, via the bandmember, to the cylindrical members in the outermost peripheral region ofthe square lattice array joined to the inner side of the band member,and then the force is sequentially transmitted to the cylindricalmembers arranged on the inner peripheral side of the square latticearray. In the case where the band member and the cylindrical members aresubstantially continuously arranged in the transmission path of theload, as described above, the joined body of the band member and thecylindrical members exhibits, as one body, an effect of ensuring thestrength, to thereby sufficiently ensure the structural strength of theentire fuel spacer.

On the contrary, in the case where the cylindrical members located atthe lattice positions associated with the short-length fuel rods in theouter peripheral region are removed, the arrangement of the cylindricalmembers in the outermost peripheral region of the square lattice arraybecomes discontinuous at the positions where the cylindrical members areremoved. This makes the strength ensuring effect of the joined body,which is composed of a large number of the cylindrical members,insufficient, and thereby reduces the structural strength of the entirefuel spacer.

To minimize such a reduction in strength, for example, Japanese PatentLaid-open No. Hei 6-3473 discloses a fuel spacer having a structure inwhich eight bath-tubs for mainly receiving forces applied from a fuelchannel box are provided on a square-shaped band member. To be morespecific, two of the bath-tubs are provided on each side of the bandmember in such a manner as to face to two of the bath-tubs provided onthe opposed side of the band member; and cylindrical members arenecessarily provided at all of the lattice positions between the two ofthe facing bath-tubs to ensure the strength, and the cylindrical memberslocated at the other lattice positions are removed to reduce thepressure loss.

The fuel spacer disclosed in Japanese Patent Laid-open No. Hei 6-3473,however, causes another problem.

That is to say, in the design of a fuel assembly including short-lengthfuel rods, the arrangement of the short-length fuel rods variesdepending on the required nuclear characteristics. However, in the abovefuel spacer, the cylindrical members located at all of the latticepositions between the two of the facing bath-tubs cannot be omitted, andaccordingly, if the short-length fuel rod is arranged at one of thelattice positions between the two of the facing bath-tubs, it fails tosufficiently reduce the pressure loss. Conversely, in the case of givingprecedence to a sufficient reduction in pressure loss, the short-lengthfuel rods cannot be arranged at all of the lattice positions between twoof the facing bathtubs, and correspondingly, the degree of freedom indesign of the fuel assembly is limited.

(2) Problem in Terms of Spring Arrangement

In the above fuel assembly in which the short-length fuel rods arearranged in the outermost peripheral region and adjacent to the waterrods, if the cylindrical members located at the lattice positionsassociated with the short-length fuel rods in the fuel spacer positionedabove the upper ends of the short-length fuel rods are simply removed,there occurs another problem.

Description of such a problem will be presented by way of example withreference to the fuel assembly shown in FIGS. 2 and 3 (described later).This fuel assembly includes fuel rods located in a square lattice arrayof 9 row×9 columns, and two water rods are arranged in a region in whichseven of the fuel rods are arrangeable. Further, in the fuel assembly,four of the short-length fuel rods are arranged one at each midpoint ofeach side of the outermost periphery of the square lattice array, andtwo of the short-length fuel rods are arranged at two corners of asquare lattice array of 3 row×3 columns at a central portion of the fuelspacer in such a manner as to be adjacent to the water rods.

It is assumed that in the fuel spacer positioned above the upper ends ofthe short-length fuel rods in the above-described fuel assembly, cellslocated at lattice positions associated with the short-length fuel rodsare simply removed, and, like the prior art manner, only one kind ofloop-shaped springs are used for eliminating an increase in the numberof parts. In this case, there occurs a requirement to entirely reviewthe arrangement of the loopshaped springs over the fuel spacer. There-arrangement of the springs is shown as a comparative example in FIG.24.

Referring to FIG. 24, in a fuel spacer 103, cells 104 in the form ofcylindrical members are bundled and joined to each other. A loop-shapedspring 105 for pressingly holding fuel rods 101 is provided at a joinedportion between a pair of the adjacent cells 104. At this time, theprovision of the cells 104 at six lattice positions 106 a, 106 b, 106 c,106 d, 106 e, and 106 f associated with the short-length fuel rods 101Ais omitted to reduce the pressure loss. As a result of omission of thecells 104, as shown in FIG. 24, the arrangement of the loop-shapedsprings 105 is entirely changed from that described in Japanese PatentLaid-open No. Hei 2-163695.

In the re-arrangement of the springs shown in FIG. 24, two of theloop-shaped springs 105A and 105B, each of which is free, that is, hasno mating cell paired therewith, are adjacently present at each of thelattice positions 106 e and 106 f, that is, at the two corners of thesquare lattice array of 3 row×3 columns in the, central portion of thefuel spacer, in such a manner as to be adjacent to the water rods 102.To allow the loop-shaped springs 105A and 105B to exhibit a springfunction, it is required to provide a spring pressing structure allowingthe loopshaped springs 105A and 105B to simultaneously generate pressingforces at each of the lattice positions 106 e and 106 f.

However, such a spring pressing structure for mounting two or more ofthe free loop-shaped springs at one lattice position has not been known.Further, it is also required to examine a connection mechanism betweenthe spring pressing structure and a water rod holding member 108, formedinto an approximately Ω-shape in transverse cross-section, for holdingthe water rod.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a fuel assemblywhich is capable of sufficiently reducing the pressure loss of a fuelspacer positioned upward from the upper ends of short-length fuel rodsirrespective of the arrangement of the short-length fuel rods andensuring the structural strength of the fuel spacer.

A second object of the present invention is to provide a fuel assemblywhich is capable of sufficiently reducing the pressure loss of a fuelspacer positioned upward from the upper ends of short-length fuel rodsirrespective of the arrangement of the short-length fuel rods andreasonably arranging the necessary minimum number of one kind ofloop-shaped springs over the fuel spacer.

(1) To achieve the above first object, according to the presentinvention, there is provided a fuel assembly including:

a plurality of fuel rods located in a square lattice array, the fuelrods including a plurality of short-length fuel rods each having a fuelactive length shorter than that of each of the remaining ones of thefuel rods;

at least one water rod arranged in a region in which one or more of thefuel rods are arrangeable; and

a plurality of fuel spacers, provided at a plurality of positions in theaxial direction, for holding the plurality of fuel rods and the at leastone water rod with mutual radial intervals thereof kept immovable;

wherein the plurality of short-length fuel rods include at least onefirst short-length fuel rod arranged in the outermost peripheral regionof the square lattice array;

each of the plurality of fuel spacers includes a plurality ofcylindrical members which are connected to each other and in which thefuel rods are to be inserted respectively, and a band member forsurrounding the outermost peripheries of the plurality of cylindricalmembers; and

the plurality of fuel spacers include first fuel spacers positionedabove the upper end of the at least one first short-length fuel rod, andat least one of the first fuel spacers is configured such that one ofthe cylindrical members, located at a first lattice position associatedwith the at least one first short-length fuel rod, is omitted, and,instead, a first supporting member for connecting two first cylindricalmembers, of the plurality of cylindrical members, adjacently located onboth sides of the first lattice position in the outermost peripheralregion adjacent to the band member, is provided at the first latticeposition.

When a plurality of fuel spacers are provided at a plurality ofpositions in the axial direction, no fuel rod is present at the firstlattice position associated with the at least one first short-lengthfuel rod in the first fuel spacer positioned above the upper end of theat least one first short-length fuel rod. Accordingly, by omitting thecylindrical member located at the first lattice position, it is possibleto reduce the flow resistance of a coolant flowing upward in the fuelassembly, and hence to sufficiently reduce the pressure loss.

The omission of the cylindrical member makes for a discontinuousarrangement of the cylindrical members located in the outermostperipheral region of the square lattice array at the first latticeposition. However, by connecting the two first cylindrical members onboth sides of the first lattice position in the outermost peripheralregion to the band member by means of the first supporting member, thetwo first cylindrical members are fixed to each other via the bandmember. With this configuration, a load transmitted from the band membercan be received by the connected structure composed of the two firstcylindrical members and the first supporting member fixedly connected toeach other.

This allows the first fuel spacer to exhibit a structural strengthsubstantially comparable to that of a fuel spacer in which thecylindrical member is present at the first lattice position. The firstsupporting member can be arranged irrespective of arrangement of thebath-tubs, and accordingly, unlike the prior art structure, even whenthe first short-length fuel rod is located at a position between the twoopposed bath-tubs in the outermost peripheral region, the cylindricalmember at the position can be removed and instead the first supportingmember can be provided thereat to sufficiently reduce the pressure loss.

As described above, it is possible to sufficiently reduce the pressureloss of the first fuel spacer positioned above the upper end of thefirst shortlength fuel rod while usually ensuring the structuralstrength of the first fuel spacer irrespective of the arrangement of thefirst short-length fuel rod.

(2) To achieve the above first object, according to the presentinvention, there is also provided a fuel assembly including:

a plurality of fuel rods arranged in a square lattice array, the fuelrods including a plurality of short-length fuel rods each having a fuelactive length shorter than that of each of the remaining ones of thefuel rods;

at least one water rod arranged in a region in which one or more of thefuel rods are arrangeable; and

a plurality of fuel spacers, provided at a plurality of positions in theaxial direction, for holding the plurality of fuel rods and the at leastone water rod with mutual radial intervals thereof kept immovable;

wherein the plurality of short-length fuel rods include at least onefirst short-length fuel rod arranged in the outermost peripheral regionof the square lattice array;

each of the plurality of fuel spacers includes a plurality ofcylindrical members which are connected to each other and in which thefuel rods are to be inserted respectively, and a band member forsurrounding the outermost peripheries of the plurality of cylindricalmembers; and

the plurality of fuel spacers include first fuel spacers positionedabove the upper end of the at least one first short-length fuel rod, andat least one of the first fuel spacers is configured such that one ofthe cylindrical members, located at a first lattice position associatedwith the at least one first short-length fuel rod, is omitted, and,instead, a second supporting member for connecting two pieces of firstcylindrical members, of the plurality of cylindrical members, adjacentlylocated on both sides of the first lattice position in the outermostperipheral region, to a second cylindrical member, of the plurality ofcylindrical members, located inwardly from and adjacently to the firstlattice position.

When a plurality of the fuel spacers are provided at a plurality ofpositions in the axial direction, no fuel rod is present at the firstlattice position associated with the at least one first short-lengthfuel rod in the first fuel spacer positioned above the upper end of theat least one first short-length fuel rod. Accordingly, by omitting thecylindrical member located at the first lattice position, it is possibleto reduce the flow resistance of a coolant flowing upward in the fuelassembly, and hence to sufficiently reduce the pressure loss.

The omission of the cylindrical member makes for a discontinuousarrangement of the cylindrical members located in the outermostperipheral region of the square lattice array at the first latticeposition. However, by connecting the two first cylindrical members onboth sides of the first lattice position in the outermost peripheralregion to the second cylindrical member located inwardly from andadjacently to the first lattice position by means of the secondsupporting member, the two first cylindrical members are fixed to eachother via the second cylindrical member. With this configuration, a loadtransmitted from the band member can be received by the connectedstructure composed of the two first cylindrical members, secondsupporting member, and second cylindrical member fixedly connected toeach other.

This allows the first fuel spacer to exhibit a structural strengthsubstantially comparable to that of a fuel spacer in which thecylindrical member is present at the first lattice position. The secondsupporting member can be arranged irrespective of arrangement of thebath-tubs, and accordingly, unlike the prior art structure, even whenthe first shortlength fuel rod is located at a position between the twoopposed bath-tubs in the outermost peripheral region, the cylindricalmember at the position can be removed and instead the second supportingmember can be provided thereat to sufficiently reduce the pressure loss.

As described above, it is possible to sufficiently reduce the pressureloss of the first fuel spacer positioned above the upper end of thefirst shortlength fuel rod while usually ensuring the structuralstrength of the first fuel spacer irrespective of arrangement of thefirst short-length fuel rod.

(3) In the configuration of the invention described in section (1) orsection (2), preferably, the first or second supporting member has atransverse cross-section smaller than that of the cylindrical member.

(4) In the configuration of the invention described in section (1) orsection (2), preferably, the at least one first fuel spacer includes, ata connection portion between one of the two first cylindrical membersand the first or second supporting member, spring pressing means forimparting a pressing force to a spring for holding the fuel rod insertedin the one of the two first cylindrical members. With thisconfiguration, it is possible to increase the degree of freedom inarrangement of the fuel rod holding springs in the fuel spacer.

(5) In the configuration of the invention described in section (1) orsection (2), preferably, the plurality of short-length fuel rods includeat least one second shortlength fuel rod arranged in a region adjacentto the at least one water rod;

each of the plurality of fuel spacers includes a water rod holdingmember connected to those of the plurality of cylindrical membersarranged in the innermost peripheral region of the square lattice arrayfor holding the at least one water rod; and

the at least one first fuel spacer is configured such that one of thecylindrical members, located at a second lattice position associatedwith the at least one second short-length fuel rod, is omitted andinstead a third supporting member for connecting two third cylindricalmembers, of the plurality of cylindrical members, located outwardly fromand adjacently to the second lattice position in the square latticearray to the water rod holding member.

(6) In the configuration of the invention described in section (1) orsection (2), preferably, each of the plurality of fuel spacers includesa plurality of first projecting members provided on the band member, thefirst projecting member projecting between the two adjacent ones of thecylindrical members in the outermost peripheral region of the squarelattice array for introducing the flow of a coolant; and

the at least one first fuel spacer is configured such that at least oneof the projecting members adjacent to the first or second supportingmember is omitted.

The provision of the first projecting members in each fuel spacer iseffective to direct the flow of a coolant in the fuel assembly towardthe fuel rod side as much as possible, and hence to improve the effectof cooling the fuel rods and enhance the critical power characteristic.In the first fuel spacer positioned above the upper ends of theshort-length fuel rods, no fuel rod is present at the first latticeposition at which the first or second supporting member is provided.Accordingly, the provision of the first projecting members in thevicinity of the first or second supporting member is not effective somuch. For this reason, by omitting the first projecting members adjacentto the first or second supporting member, it is possible to reduce anincrement of pressure loss due to the projecting shapes thereof, andhence to further reduce the pressure loss.

(7) In the configuration of the invention described in section (6),preferably, the at least one first shortlength fuel rod is arranged inthe outermost peripheral region except for four corners of the squarelattice array. The effect of improving the critical power characteristicis largest at the four corners in the outermost peripheral region of thesquare lattice array, and becomes smaller at other positions in theoutermost peripheral region.

Accordingly, in the case where the first or second supporting member isarranged at a lattice position associated with the first short-lengthfuel rod, other than the four corners in the outermost peripheralregion, the effect of improving the critical power characteristic is notreduced so much even by omitting the projecting members adjacent to thesupporting member. That is to say, the omission of the projectingmembers is more effective in the case where the first or secondsupporting member is arranged at a lattice position associated with thefirst short-length fuel rod, other than at the four corners in theoutermost peripheral region.

(8) To achieve the above second object, according to the presentinvention, there is provided a fuel assembly including:

a plurality of fuel rods arranged in a square lattice array, the fuelrods including a plurality of short-length fuel rods each having a fuelactive length shorter than that of each of the remaining ones of thefuel rods;

at least one water rod arranged in a region in which one or more of thefuel rods are arrangeable; and

a plurality of fuel spacers, provided at a plurality of positions in theaxial direction, for holding the plurality of fuel rods and the at leastone water rod with mutual radial intervals thereof kept immovable;

wherein the plurality of short-length fuel rods include at least onefirst short-length fuel rod arranged in the outermost peripheral regionof the square lattice array, and at least one second short-length fuelrod arranged at a lattice position adjacent to the at least one waterrod;

each of the plurality of fuel spacers includes a plurality ofcylindrical members which are connected to each other and in which thefuel rods are to be inserted respectively, and first loop-shaped springseach of which is provided at a joined portion between a pair of theadjacent ones of the plurality of cylindrical members for pressing twoof the fuel rods inserted in the adjacent cylindrical members;

the plurality of fuel spacers include first fuel spacers positionedabove the upper ends of the first and second shortlength fuel rods, andat least one of the first fuel spacers is configured such that those ofthe plurality of cylindrical members located at first and second latticepositions associated with the first and second short-length fuel rodsare omitted;

each of those of the plurality of cylindrical members located at latticepositions adjacent to the second lattice position has on the secondlattice position side a second loop-shaped spring for pressing the fuelrod in the cylindrical member; and

the plurality of second loop-shaped springs located at the latticepositions adjacent to the second lattice position are supported by aspring pressing member provided at the second lattice position.

In the at least one first fuel spacer positioned above the upper ends ofthe first and second shortlength fuel rods, the pressure loss can bereduced by omitting the unnecessary cylindrical members located at thefirst and second lattice positions.

As a result of removal of the cylindrical members located at the firstand second lattice positions in the first fuel spacer, there may be arequirement for reviewing the arrangement of the loop-shaped springsover the fuel spacer. In this case, there may be often provided a secondloop-shaped spring of the same kind as that of the first loop-shapedspring on the second lattice side of each of a plurality of thecylindrical members located at lattice positions adjacent to the secondlattice position for pressing the fuel rod in the cylindrical member.

Incidentally, the loop-shaped spring is generally configured such thatit does not generate any pressing force in the state in which the fuelrods are not inserted in the adjacent cylindrical members between whichthe loop-shaped spring is mounted. In this regard, each of the pluralityof second loop-shaped springs is freely movable on the second latticeposition side because no fuel rod is present at the second latticeposition, and therefore, it does not generate any pressing force. Tocope with such an inconvenience, according to the present invention, thespring pressing member is provided at the second lattice position. Thespring pressing member supports the plurality of free second loop-shapedsprings in such a manner that the second loop-shaped springs generatepressing forces applied to the associated fuel rods.

(9) In the configuration of the invention described in section (8),preferably, the spring pressing member in the at least one first fuelspacer includes a plurality of spring holding portions which areinserted in the loops of the plurality of second loop-shaped springs forholding the plurality of second loop-shaped springs respectively; and aplurality of spring pressing portions which are brought in contact withthe loops of the plurality of second loopshaped springs from the outerperipheral side for supporting the plurality of second loop-shapedsprings such that the plurality of second loop-shaped springs generatepressing forces applied to the associated ones of the fuel rodsrespectively.

(10) In the configuration of the invention described in section (9),preferably, the plurality of spring holding portions include a pluralityof spring holding projecting pieces and the plurality of spring pressingportions include a plurality of spring pressing projecting pieces; and

the plurality of spring holding projecting pieces all project in onedirection, and at least one of the plurality of spring pressingprojecting pieces projects in the opposite direction.

The spring holding projecting pieces and the spring pressing projectingpieces of the spring pressing member are generally formed by cutting thebase plate portion of the spring pressing member into tongue shapes. Thespring pressing projecting pieces are brought in contact with the loopsof the second loop-shaped springs from the outer peripheral side, sothat the spring pressing projecting pieces are required to project onthe inner side of the spring pressing member more than the springholding projecting pieces inserted in the loops of the loop-shapedsprings. Accordingly, if the projecting direction of all of the springpressing projecting pieces is made identical to the projecting directionof the spring holding projecting pieces, the cut-in amount along boththe sides of each of the spring pressing projecting pieces must be madelarger, and correspondingly, the width of a portion, which is equivalentto the non-cut root portion of the spring pressing projecting piece, ofthe spring pressing member becomes smaller. This makes it difficult toensure sufficient strength and rigidity against the pressing forces ofthe second loop-shaped springs.

To cope with such a problem, according to the present invention, theprojecting direction of at least one of the spring pressing projectingpieces is opposed to the projecting direction of the spring holdingprojecting pieces. This is effective to solve such a problem and toensure sufficient strength and rigidity.

(11) In the configuration of the invention described in section (9),preferably, the plurality of spring holding portions include a pluralityof spring holding projecting pieces and the plurality of spring pressingportions include a plurality of spring pressing projecting pieces; and

at least one of the plurality of spring pressing projecting pieces isconfigured such that the leading end thereof is connected to a portion,which is opposed to the leading end of the spring pressing projectingpiece, of a base plate portion of the spring pressing member.

Since at least one of the spring pressing projecting pieces isconfigured such that the root portion and leading end thereof areintegrated with the base plate portion, it is possible to ensuresufficient strength and rigidity against the pressing forces of thesecond loop-shaped springs.

(12) In the configuration of the invention described in section (8),preferably, the at least one first fuel spacer further includes a waterrod holding member for holding the at least one water rod in the radialdirection; and the spring pressing member is joined to the water rodholding member. The joined body composed of the spring pressing memberand the water rod holding member can support the second loop-shapedsprings so as to be free on the second lattice position side such thatthe second loop-shaped springs generate pressing forces applied to thefuel rods, and also it can hold the at least one water rod in the radialdirection.

(13) In the configuration of the invention described in section (8),preferably, the spring pressing member of the at least one first fuelspacer serves as a water rod holding member for holding the at least onewater rod in the radial direction.

(14) In the configuration of the invention described in section (8),preferably, the plurality of second loopshaped springs are provided onthe second lattice sides of two of the plurality of cylindrical members,positioned adjacent to each other on the second lattice in the rowdirection and column direction.

(15) To achieve the above second object, according to the presentinvention, there is also provided a fuel assembly including:

a plurality of fuel rods arranged in a square lattice array of 9 rows×9columns, the fuel rods including a plurality of short-length fuel rodseach having a fuel active length shorter than that of each of theremaining ones of the fuel rods;

two water rods arranged in a region within an array of 3 rows×3 columnsof the square lattice array, in which region seven pieces of the fuelrods are arrangeable; and

a plurality of fuel spacers, provided at a plurality of positions in theaxial direction, for holding the plurality of fuel rods and the waterrods with mutual radial intervals kept immovable;

wherein the plurality of short-length fuel rods include four firstshort-length fuel rods each being arranged at the midpoint on each sideof a square shape formed by the outermost peripheral region of thesquare lattice array, and two second short-length fuel rods arranged ina region of the array of 3 rows×3 columns except for the region in whichthe two water rods are arranged;

each of the plurality of fuel spacers includes a plurality ofcylindrical members which are connected to each other and in which thefuel rods are to be inserted respectively, and first loop-shaped springseach of which is provided at a joined portion between a pair of theadjacent ones of the plurality of cylindrical members for pressing twoof the fuel rods inserted in adjacent cylindrical members;

the plurality of fuel spacers include first fuel spacers positionedabove the upper ends of the first and second short-length fuel rods, andat least one of the first fuel spacers is configured such that four ofthe cylindrical members located at four first lattice positionsassociated with the four first short-length fuel rods and two of thecylindrical members located at two second lattice positions associatedwith the two second short-length fuel rods are omitted;

each of two of the plurality of cylindrical members, located at latticepositions adjacent to each of the second lattice positions in the rowdirection and column direction, has on the second lattice position sidea second loop-shaped spring for pressing the fuel rod in the cylindricalmember; and

the two second loop-shaped springs located at the two lattice positionsadjacent to the second lattice position are supported by a springpressing member provided at the second lattice position.

(16) In the configuration of the invention described in section (15),preferably, the spring pressing member has at least one secondprojecting member which projects outward from the outer peripheral sideof the spring pressing member for introducing the flow of a coolant inthe projecting direction of the second projecting member.

The spring pressing member at the second lattice position, which is at alevel at which no fuel rod is present, is not required to be cooled.Accordingly, to direct the flow of a coolant passing through the springpressing member toward the other fuel rods around the lattice positionas much as possible, the second projecting members are provided on thespring pressing member. This makes effective use of the coolant andhence improves the effect of cooling the fuel rods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view showing the structure of a fuel spacer positionedupward from the upper ends of shortlength fuel rods in a fuel assemblyaccording to a first embodiment of the present invention;

FIG. 2 is a vertical sectional view showing the structure of the fuelassembly to which the fuel spacer shown in FIG. 1 is applied;

FIG. 3 is a transverse sectional view taken on line A—A of FIG. 2;

FIG. 4 is a top view showing the structure of a fuel spacer positioneddownward from the upper ends of the shortlength fuel rods, whichstructure is applied to the fuel assembly according to the firstembodiment;

FIG. 5 is a perspective view showing the structure of a supportingmember in the fuel spacer shown in FIG. 1;

FIG. 6 is a top view showing a modification of the fuel spacer shown inFIG. 1, in which lattice positions associated with the firstshort-length fuel rods are located between two opposed bath-tubs;

FIG. 7 is a top view showing the structure of a fuel spacer according toa second embodiment of the present invention;

FIG. 8 is a top view showing a modification of the fuel spacer shown inFIG. 7, in which supporting members, each of which is composed of anapproximately cylindrical shape similar to that of a cell, are provided;

FIG. 9 is a top view showing the structure of a fuel spacer according toa third embodiment of the present invention;

FIG. 10 is a top view showing a modification of the fuel spacer shown inFIG. 9, in which supporting members are provided at lattice positionsassociated with second short-length fuel rods and spring supportingmembers are removed from supporting members located at lattice positionsassociated with first short-length fuel rods;

FIG. 11 is a top view showing a modification of the fuel spacer shown inFIG. 9, in which a supporting member connected to a water rod holdingplate is formed into a shape similar to that of a supporting memberlocated at a lattice position associated with the first short-lengthfuel rod;

FIG. 12 is a top view showing the structure of a fuel spacer accordingto a fourth embodiment of the present invention;

FIG. 13 is a top view showing a modification of the fuel spacer shown inFIG. 1, which is applied to a fuel assembly including a square-shapedwater rod;

FIG. 14 is a top view showing a fuel spacer positioned upward from theupper ends of short-length fuel rods in a fuel assembly according to afifth embodiment of the present invention;

FIG. 15 is a transverse sectional view showing the detailed structure ofthe fuel spacer shown in FIG. 14;

FIG. 16 is a perspective view showing the structure near the joinedportions between a water rod holding member and cells in the fuel spacershown in FIG. 15;

FIG. 17 is a perspective view, with parts partially cutaway, showing thestructure near the joined portions between a spring pressing member andcells in the fuel spacer shown in FIG. 14;

FIG. 18 is a perspective view, with parts partially cutaway, showing thestructure near the joined portions between a spring pressing member andcells in a comparative example;

FIG. 19 is a perspective view, with parts partially cutaway, showing thestructure near the joined portions between a spring pressing member andcells in a first modification of the structure shown in FIG. 17;

FIG. 20 is a perspective view, with parts partially cutaway, showing thestructure near the joined portions between a spring pressing member andcells in a second modification of the structure shown in FIG. 17;

FIG. 21 is an enlarged sectional view showing a modification of thestructure shown in FIG. 14, in which flow tabs are provided on a springpressing member;

FIG. 22 is a top view showing the structure of a fuel spacer accordingto a sixth embodiment of the present invention;

FIG. 23 is a top view showing an essential structure of a modificationof the fuel spacer shown in FIG. 14 or 22, in which three or moreloop-shaped springs are supported by a spring supporting member; and

FIG. 24 is a top view illustrating problems caused in a fuel spacerpositioned upward from the upper ends of short-length fuel rods in afuel assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

First Embodiment

A first embodiment of the present invention will be described withreference to FIGS. 1 to 6. A vertical sectional view showing thestructure of a fuel assembly in this embodiment is shown in FIG. 2, anda transverse sectional view taken on line A—A of FIG. 2 is shown in FIG.3.

Referring to FIGS. 2 and 3, a fuel assembly 1 includes 74 fuel rods 2,filled with fuel pellets (not shown), which fuel rods are arranged in asquare lattice array of 9 rows×9 columns; two water rods 3 arranged in aregion in which seven of the fuel rods 2 are arrangeable; fuel spacers 4for holding the fuel rods 2 and the water rods 3 with mutual radialintervals thereof kept immovable; an upper tie plate 5 and a lower tieplate 6 for holding the upper end portion and the lower end portion of afuel bundle composed of the fuel rods 2 and the water rods 3,respectively; and a fuel channel box 8 for covering the outer peripheralportion of the above structure.

The fuel rods 2 include normal-length fuel rods 2 a, each having anormal fuel active length (filling length of fuel pellets), andshort-length fuel rods 2 b each having an effective length shorter thanthat of the normal-length fuel rods 2 a. The short-length fuel rods 2 binclude four first short-length fuel rods 2 b 1 arranged in theoutermost peripheral region of the square lattice array, and two secondshort-length fuel rods 2 b 2 arranged in a region adjacent to the waterrods 3.

The fuel spacers 4 are provided at a plurality of positions arranged inthe axial direction. As shown in FIG. 3, there are 74 fuel rods 2,including the short-length fuel rods 2 b, which are short in fuel activelength and accordingly, the lattice positions, which are filled with theshortlength fuel rods 2 b in the fuel spacer 4 a positioned on the lowerportion of the fuel assembly 1, become empty of fuel rods in the fuelspacer 4 b positioned on the upper portion of the fuel assembly 1. Forthis reason, the structure of the fuel spacer 4 b positioned above theupper ends of the short-length fuel rods 2 b is designed to be slightlydifferent from the structure of the fuel spacer 4 a positioned below theupper ends of the short-length fuel rods 2 b. Top views of thestructures of these fuel spacers 4 a and 4 b are shown in FIGS. 4 and 1,respectively.

Referring to FIGS. 4 and 1, each of the fuel spacers 4 a and 4 bincludes a large number (74 cells for the spacer 4 a, 70 cells for thespacer 4 b) of cylindrical members (cells) 9 which are arranged in asquare lattice array of 9 rows×9 columns corresponding to the squarelattice array of the fuel rods 2, and these cylindrical members arewelded to each other and are of a size to permit the fuel rods 2 to beinserted therein, respectively; a square-shaped band member (band) 11which surrounds the outer periphery of the joined cells 9; water rodholding members 12, each being formed into a Ω-shape in transversecross-section, which are welded to those cells, which are arranged inthe innermost peripheral region of the square lattice array, of thecells 9, for holding the water rods 3 in the radial and axialdirections; approximately quarter-round water rod holding members 13;and water rod holding springs 14, provided on the water rod holdingmembers 13, for imparting pressing forces to hold the water rods 3 inposition.

Each cell 9, which is formed into an approximately cylindrical shape,includes two projections 9 a for holding a respective fuel rod 2; and aspring supporting portion (not shown), provided at the joined portionwith the adjacent cell 9, for suitably supporting a loop-shaped spring10 to press against the fuel rod 2 inserted in the cell 9. It should benoted that the structures of the loop-shaped spring and the springsupporting portion, while not shown particularly in detail in thefigures, are known for example from Japanese Patent Laid-open No. Hei6-273560.

The band 11, having a square-shape, whose four sides are welded to eachother, includes a large number of flow tabs 15 each of which is bent insuch a manner as to project between the adjacent ones of the cells 9 inthe outermost peripheral region of the square lattice array in order tointroduce the flow of a coolant; and eight bathtubs 16 provided two foreach side of the square-shape of the band 11, each tub projecting on thefuel channel box 8 side so as to be brought in contact with the innersurface of the fuel channel box 8.

The feature of this embodiment lies in the structure of the fuel spacer4 b. That is to say, the fuel spacer 4 b shown in FIG. 1 is differentfrom the fuel spacer 4 a shown in FIG. 4 in that the cells 9 located atfirst lattice positions 7 a associated with the first short-length fuelrods 2 b 1 are omitted and instead the supporting members 17 areprovided at the lattice positions 7 a. The supporting member 17 connectsthe two cells 9A and 9B, adjacently located on both the sides of thefirst lattice position 7 a associated with the first short-length fuelrod 2 b 1 in the outer peripheral region of the square lattice array, tothe band 11.

FIG. 5 is a perspective view showing the structure of the supportingmember 17. Referring to FIG. 5, the supporting member 17 is formed intoa shape similar to one of two halves obtained by vertically dividing acylinder having an octagonal cross-section. While not shown in FIG. 5 toavoid complication in the drawing, as shown in FIG. 1, the supportingmember 17 includes, at the joined portion with the adjacent cell 9A, aspring supporting portion for suitably supporting the loop-shaped spring10, which operates to hold the fuel rod 2 inserted in the cell 9A byimparting a pressing force against the fuel rod 2. It should be notedthat the structure of the spring supporting portion, while not shownparticularly in detail in the figures, is known for example fromJapanese Patent Laid-open No. Hei 2-163695.

In the fuel spacer 4 b, the cells 9 located at the lattice positionsassociated with the second short-length fuel rods 2 b 2 are left as theyare; however, the loop-shaped springs 10, which are unnecessary for thecells 9, because the fuel rods are not inserted in the cells 9, areremoved from the cells 9.

The fuel assembly in this embodiment, which is configured as describedabove, exhibits the following effects:

(1) Reduction in Pressure Loss

This effect will be described with reference to a comparative example inwhich a fuel spacer having the same structure as that of the fuel spacer4 a, in which all of the cells 9 are located without any being omittedat all of the lattice positions, as shown in FIG. 4, is positioned abovethe upper ends of the first short-length fuel rods 2 b 1 of the fuelassembly 1. In this comparative example, the cells 9, which are notrequired to be provided at the lattice positions associated with thefirst short-length fuel rods 2 b 1 because the fuel rods are not presentat the lattice positions, are provided at the lattice positions, andtherefore, the pressure loss is correspondingly increased.

On the contrary, in the fuel spacer 4 b in this embodiment, the cells 9at the first lattice positions 7 a associated with the firstshort-length fuel rods 2 b 1 are omitted, and instead the supportingmembers 17 each being formed into a semi-octagonal cross-sectionalshape, are provided at the first lattice positions 7 a, as shown in FIG.1. As a result, since the flow resistance of water as a coolant flowingupward in the fuel assembly 1 is made significantly smaller than that inthe comparative example, it is possible to sufficiently reduce thepressure loss.

(2) Attainment of Structural Strength

This effect will be described in detail with reference to theabove-described comparative example. As described above, in the fuelspacer (having the same structure as that of the fuel spacer 4 a shownin FIG. 4) in the comparative example, all of the cells 9 in contactwith the band 11 surrounding the outer periphery of the spacer arecontinuously in contact with each other, to thereby maintain thestructural strength of the entire spacer.

For example, if an external force is applied to the fuel spacer via thefuel channel box 8 in case of an earthquake or in handling the fuelassembly, the load is first transmitted to eight of the bath-tubs 16provided on the band 11. After that, the load is transmitted, via theband 11, to the cells 9 in the outermost peripheral region of the squarelattice array joined to the inner side of the band 11, and then theforce is sequentially transmitted to the cells 9 arranged on the innerperipheral side of the square lattice array (see FIG. 4). In this way,for the fuel spacer in this comparative example, since the cells 9arranged in succession in the path along which the load is transmittedare integrally formed and an integrity of ensuring the structural effectas a whole is obtained, it is possible to sufficiently ensure thestructural strength of the entire fuel spacer.

On the contrary, for the fuel spacer 4 b in this embodiment, as shown inFIG. 1, since one cell 9 between the cells 9A and 9B at each side of thesquare lattice array is omitted, the arrangement of the cells 9 in theoutermost peripheral region becomes discontinuous at the positionbetween the cells 9A and 9B. In this embodiment, however, since the twocells 9A and 9B are joined to each other by means of the supportingmember 17, the cells 9A and 9B are rigidly fixed to each other via theband 11. As a result, when a load is transmitted from the band 11 asdescribed above, it can be received by the joined structure composed ofthe fixed two cells 9A and 9B and the supporting member 17, andaccordingly, it is possible for the fuel spacer 4 b to provide astructural strength substantially comparable to that of the fuel spacerin the comparative example in which a cell 9 is located at the latticeposition between the cells 9A and 9B.

(3) Attainment of Degree of Freedom in Design of Short-Length Fuel RodArrangement

As described in the paragraphs (1) and (2), the fuel spacer 4 b in thisembodiment is effective to reduce the pressure loss while ensuring thestructural strength of the entire spacer. Such an effect can be obtainedeven if the lattice position associated with the first short-length fuelrod 2 b 1 is located at any position in the outermost peripheral regionof the square lattice array. In other words, according to thisembodiment, it is possible to ensure the degree of freedom in design.

In the design of a fuel assembly including shortlength fuel rods, asdescribed above, various arrangements of the short-length fuel rods maybe considered in accordance with the nuclear characteristics requiredfor the fuel assembly. Therefore, for example, there may be consideredan arrangement of the first short-length fuel rod 2 b 1 at a positionbetween the two opposed bath-tubs 16 in the outermost peripheral regionof the square lattice array.

In this case, for example, in the fuel space having the prior artstructure disclosed in Japanese Patent Laid-open No. Hei 6-3473, sincethe cells 9 at all of the lattice positions between the two opposedbath-tubs 16 cannot be omitted, the pressure loss cannot be sufficientlyreduced. On the other hand, if the reduction in pressure loss takesprecedence, the short-length fuel rods cannot be arranged at all of thelattice positions between the two opposed bath-tubs 16, and therefore,the degree of freedom in design of the fuel assembly is correspondinglylimited.

On the contrary, in such a case, the fuel spacer 4 b in this embodimentcan be modified, in accordance with the arrangement of the latticepositions associated with the first short-length fuel rods 2 b 1, forexample, into a fuel spacer 4 bA shown in FIG. 6 in which the cells 9located at the lattice positions between the two opposed bath-tubs 16are omitted and instead the supporting members 17 are provided at thoselattice positions. Accordingly, unlike the fuel spacer having the priorart structure, even if the lattice positions associated with the firstshort-length fuel rods 2 b 1 are located between the two opposedbath-tubs 16 in the outermost peripheral region of the square latticearray, it is possible to sufficiently reduce the pressure loss whileensuring the strength of the fuel spacer.

As described in the paragraphs (1) to (3), according to the fuel spacer4 b in this embodiment, it is possible to sufficiently reduce thepressure loss of the fuel spacer 4 b positioned upward from the upperends of the short-length fuel rods 2 b 1 while usually ensuring thestructural strength of the fuel spacer 4 b irrespective of thearrangement of the lattice positions associated with the firstshort-length fuel rods 2 b 1.

(4) Attainment of Degree of Freedom in Design of Spring Arrangement

As described above, the known loop-shaped spring 10 for pressing thefuel rods 2 is essentially disposed between the adjacent cells 9, and itfunctions to generate pressing forces when the fuel rods 2 are insertedin the cells 9, respectively. Accordingly, if the means for imparting aspring pressing force is not provided on the supporting member 17, whichis additionally provided at the lattice position associated with thefirst short-length fuel rod 2 b 1, the supporting member 17 side of theloopshaped spring 10 disposed at the joined portion between thesupporting member 17 and the adjacent cell 9A comes into a free end,with a result that the loop-shaped spring 10 cannot achieve the functionof pressing against the fuel rod 2 inserted in the cell 9A.

Accordingly, to press the fuel rod 2 in the cell 9A, the loop-shapedspring 10 must be disposed between the cell 9A and the cell 9 which isadjacent to a portion, opposed to the supporting member 17, of the cell9A. As a result, the spring arrangement in the entire spacer must bereviewed as a whole. This imposes a large limitation on the design.

In this embodiment, however, the spring supporting portion provided onthe supporting member 17 supports the loop-shaped spring 10 for pressingthe fuel rods 2 and imparts a pressing force to the loop-shaped spring10. As a result, since the loop-shaped spring 10 in the cell 9A isallowed to function just as in the fuel spacer 4 a shown in FIG. 4, itis possible to increase the degree of freedom in arrangement of theloop-shaped springs 10 and hence to ensure a degree of freedom of designcomparable to that in the fuel spacer 4 a shown in FIG. 4.

Second Embodiment

A second embodiment of the present invention will be described withreference to FIGS. 7 to 11. This embodiment has a feature such that, inthe fuel spacer positioned above the upper ends of the firstshort-length fuel rods 2 b 1, the spring arrangement and the shape ofthe supporting member are changed.

FIG. 7 is a top view showing the structure of a fuel spacer 204 b inthis embodiment. In the fuel spacer 204 b, parts common to those in thefuel spacer 4 b described in the first embodiment with reference to FIG.1 are designated by the same symbols and an explanation thereof will beomitted.

The fuel spacer 204 b shown in FIG. 7 is different from the fuel spacer4 b shown in FIG. 1 in that the cells 9 at the second lattice positions7 b associated with the second shortlength fuel rods 2 b 2 are omittedand instead supporting members 218 are provided at the second latticepositions 7 b. The supporting member 218 connects the two cells 9C and9D, located outwardly from and adjacently to the second lattice position7 b in the square lattice array, to the water rod holding member 12. Thesupporting member 218 is formed into an approximately polygonalcylindrical shape with an unnecessary side portion in terms of structurecut off for making the pressure loss as small as possible.

In the fuel spacer 204 b, a supporting member 217, having a structure inwhich the spring supporting portion is removed from the supportingmember 17 shown in FIG. 1, is used as a supporting member for connectingthe two cells 9A and 9B adjacently located on both sides of the latticeposition associated with the first short-length fuel rod 2 b 1, to theband 11. With this configuration, since a loop-shaped spring 10 is notdisposed on the supporting member 217, the spring arrangement in theentire fuel spacer is changed such that the supporting member 218 hastwo spring supporting portions (not shown) for suitably supporting thetwo loop-shaped springs 10 to press against the fuel rods 2 inserted inthe cells 9C and 9D to impart pressing forces thereto. To be morespecific, two of the known spring supporting portions having the samestructure as that of the spring supporting portions used for the cells 9are simply provided at a joined portion between the cells 9C and 9D ofthe supporting member 218. The remaining configuration of thisembodiment is substantially the same as that of the first embodiment.

According to this embodiment, in addition to the same effect as that ofthe first embodiment, there can be obtained an effect of simplifying thestructure because the supporting member 217 has no spring supportingportion.

While the supporting member 217 formed into a semi-octagonal cylindricalshape is used in the second embodiment, the present invention is notlimited thereto. For example, the supporting member 217 may be formedinto another shape, for example, a semi-cylindrical shape (with apartial peripheral length portion cut off) having the same thickness asthat of the cell 9. This exhibits the following effect.

In general, the cell 9 is manufactured by cutting a circular tube,having a specific outside diameter and a specific thickness, into aspecific length, and processing the cut piece to form the projections 9a and also cutouts for the spring supporting portion. Here, if asupporting member 217A formed into a semi-cylindrical shape having thesame thickness as that of the cell 9 is used as the supporting member,such a supporting member 217A can be manufactured using the raw circulartube for forming the cell 9. This is effective to reduce themanufacturing cost by making the circular tube shareable between thesupporting member 217A and the cell 9. From the viewpoint of reductionin pressure loss, it may be Desirable that the peripheral length of thecylindrical shape of the supporting member 217A be made as short aspossible within a length range required for welding the supportingmember 217A to the adjacent cells 9 with no problem.

Further, a supporting member 217A formed into an approximatelycylindrical shape similar to that of the cell 9 may be used as thesupporting member. FIG. 8 is a top view showing the structure of a fuelspacer 204 bA including such a supporting member 217A. The supportingmember 217A connects the cells 9A and 9B, adjacently located on bothsides of the lattice position 7 a associated with the first short-lengthfuel rod 2 b 1, to the band 11, and also the supporting member 217A isconnected to the cell 9E located inwardly from and adjacently to thefirst lattice position 7 a in the square lattice array.

In addition, the supporting member 217 is made as thin as possiblewithin an allowable thickness range in terms of the structural strengthof the fuel spacer for making the cross-sectional area smaller than thatof the cell 9 thereby reducing the pressure loss.

The supporting member 217A formed into an approximately cylindricalshape is manufactured using a circular tube having a specific thickness,which tube is different from the raw circular tube for forming the cell9, or using the raw circular tube for forming the cell 9, and grindingthe inner surface of the tube to increase the inside diameter (that is,decrease the thickness). In the latter case, there can be obtained aneffect of reducing the manufacturing cost by making the circular tubeshareable between the supporting member 217A and the cell 9.

It should be noted that the cross-sectional shape of the supportingmember 217A in this modification is not limited to a cylindrical shape,but may be of course a polygonal shape insofar as it satisfies therequirement that the cross-section of the supporting member 217A issmaller than that of the cell 9.

Further, the supporting member 217A may be configured as a member havingthe same cross-sectional shape as that of the supporting member 218except that the spring supporting portions for supporting theloop-shaped springs 10 are not provided. In the manufacture of thesupporting member 218, the member 217 (replaced from the supportingmember 217A) having the same cross-sectional shape as that of thesupporting member 218 can be manufactured by punching or bending thesame raw material as that for the supporting member 218. This iseffective to reduce the manufacturing cost by making the raw materialshareable between the member (replaced from the supporting member 217A)and the supporting member 218. In this sharing of the raw material, theshape of the supporting member 218 is not limited to a polygonalcylindrical shape but may be of course a thin cylindrical shape or acylindrical shape with a partial peripheral portion cut off.

Third Embodiment

A third embodiment of the present invention will be described withreference to FIGS. 9 to 11. This embodiment has a feature such that, inthe fuel spacer positioned above the upper ends of the firstshort-length fuel rods 2 b 1, the shape of the supporting member and thesupporting structure are further changed. FIG. 9 is a top view showingthe structure of a fuel spacer 304 b in this embodiment. In the fuelspacer 304 b, parts common to those of the fuel spacer 4 b described inthe first embodiment with reference to FIG. 1 are designated by the samesymbols and an explanation thereof will be omitted.

The fuel spacer 304 b shown in FIG. 9 is different from the fuel spacer4 b shown in FIG. 1 in that the supporting member 17 located at thefirst lattice position (see FIG. 3) associated with the firstshort-length fuel rod 2 b 1 is replaced with a supporting member 317.The supporting member 317 connects the two cells 9A and 9B adjacentlylocated on both sides of the first lattice position 7 a associated withthe first short-length fuel rod 2 b 1 in the outermost peripheral regionof the square lattice array, and the cell 9E located inwardly from andadjacently to the first lattice position 7 a in the square latticearray, to each other. In summary, the supporting member 317, which isnot joined to the band 11, joins the adjacent cells 9A, 9B and 9E toeach other.

With this configuration, the entire fuel spacer has a structure as shownin FIG. 9 in which each cell 9 in the second layer from the outermostperiphery of the square lattice array is fixedly joined to four adjacentcells or three adjacent cells and one supporting member 317 at fourpositions spaced at intervals of 90° in the circumferential directionthereof. Like the supporting member 17 in the first embodiment, thesupporting member 317 is formed into one of two halves obtained byvertically dividing a cylinder having an octagonal cross-section. Whilenot shown in detail, like the supporting member 17, the supportingmember 317 includes, at the joined portion with the adjacent cell 9A, aspring supporting portion for suitably supporting a loop-shaped spring10 to hold the fuel rod 2 inserted in the cell 9A to impart a pressingforce thereto. The other configuration is substantially the same as thatof the first embodiment.

Like the fuel assembly in the first embodiment, the fuel assembly inthis embodiment, configured as described above, has the following foureffects:

(1) Reduction in Pressure Loss

In the fuel spacer 304 b in this embodiment, the cells 9 at the firstlattice positions 7 a associated with the first short-length fuel rods 2b 1 are omitted, and instead the supporting members 317 each beingformed into a semi-octagonal cross-sectional shape are provided at thefirst lattice positions 7 a. As a result, since the flow resistance ofwater as a coolant flowing upward in the fuel assembly 1 issignificantly reduced, it is possible to sufficiently reduce thepressure loss.

(2) Attainment of Structural Strength In the fuel spacer 304 b in thisembodiment, as shown in FIG. 9, since one cell 9 between the cells 9Aand 9B at each side of the square lattice array is omitted, thearrangement of the cells 9 in the outermost peripheral region becomesdiscontinuous at the position between the cells 9A and 9B. In thisembodiment, however, since the two cells 9A and 9B are connected to theadjacent cell 9E on the inner peripheral side by means of the supportingmember 317, the two cells 9A and 9B are rigidly fixed to each other viathe cell 9E. As a result, when a load is transmitted from the band 11,it can be received by the joined structure composed of the fixedlyconnected two cells 9A and 9B, the supporting member 317, and the cell9E. Accordingly, it is possible for the fuel spacer 304 b to provide astructural strength substantially comparable to that of the fuel spacerin which the cell 9 is located at the lattice position between the cells9A and 9B.

(3) Attainment of Degree of Freedom in Design of Short-length Fuel RodArrangement

In the fuel spacer 304 b in this embodiment, even if the latticeposition associated with the first short-length fuel rod 2 b 1 islocated at any position in the outermost peripheral region of the squarelattice array, the cell 9 at the lattice position can be omitted andinstead the supporting member 317 can be provided at the latticeposition. Accordingly, unlike the fuel spacer disclosed for example inJapanese Patent Laid-open No. Hei 6-3473, even if the lattice positionsassociated with the first short-length fuel rods 2 b 1 are locatedbetween the two opposed bath-tubs 16 in the outer peripheral region ofthe square lattice array, it is possible to omit the cells 9 at thelattice positions, and hence to sufficiently reduce the pressure losswhile ensuring the strength of the fuel spacer.

(4) Attainment of Degree of Freedom in Design of Spring Arrangement

In the fuel spacer 304 b in this embodiment, the spring supportingportion provided on the supporting member 317 supports the loop-shapedspring 10 for pressing against the fuel rods 2 to impart a pressingforce thereto. Accordingly, it is possible to ensure a degree of freedomin the spring arrangement comparable to that in the fuel spacer in whichthe cells 9 are located at the first lattice positions 7 a associatedwith the first shortlength fuel rods 2 b 1.

It should be noted that the third embodiment may be variously modifiedwithout departing from the basic configuration thereof. Somemodifications will be described below.

FIG. 10 is a top view showing the structure of a fuel spacer 304 bA inwhich, like the second embodiment, supporting members 318 are providedat lattice positions associated with the second short-length fuel rods 2b 2 and further the spring supporting portions are removed from thesupporting members 317.

As shown in FIG. 10, the supporting member 318 is provided to connectthe two cells 9C and 9D, adjacently located on the outer peripheral sideof the second lattice position 7 b associated with the secondshort-length fuel rod 2 b 2 in the square lattice array, to the waterrod holding member 12; and it is formed into an approximatelycylindrical shape. Also, since supporting members 317A have no springsupporting portions, the spring arrangement in the entire spacer ischanged such that the supporting member 318 has two spring supportingportions for suitably supporting the two loop-shaped springs 10 to pressthe fuel rods 2 inserted in the cells 9C and 9D for imparting pressingforces thereto (like the second embodiment, the known two springsupporting portions are simply provided). This modification is effectiveto simplify the structure because the supporting members 317 have nospring supporting portions.

In the fuel spacer in the modification shown in FIG. 10, the supportingmembers 317A may be formed into a semi-cylindrical (with a partialperipheral length portion cut off) shape having the same thickness asthat of the cell 9. In this case, the supporting member 317A can bemanufactured using the raw circular tube for forming the cell 9. This iseffective to reduce the manufacturing cost by making the circular tubeshareable between the supporting member 317A and the cell 9. From theviewpoint of reduction in pressure loss, it may be desirable that theperipheral length of the cylindrical shape of the supporting member 317Abe made as short as possible within a length range required for weldingthe supporting member 317A to the adjacent cells 9 with no problem.

The shape of the supporting members 317A and 318A may be made similar tothat of the supporting member 318. FIG. 11 is a top view showing thestructure of a fuel spacer 304 bB including supporting members 317B and318A having the configuration described above. As shown in FIG. 11, thesupporting members 317B and 318A, each being formed into a cylindricalshape with a partial peripheral length portion cut off, are identical incross-sectional shape to each other. These supporting members 317B and318A are substantially similar to each other except that the supportingmember 317B has no spring supporting portions for supporting theloop-shaped springs 10 to press against the fuel rods 2 inserted in thecells 9C and 9D. As a result, in the manufacture of the supportingmember 318A, the supporting member 317B having the same cross-sectionalshape in this modification can be manufactured by punching or bendingthe same raw material element as that for the supporting member 318A.This is effective to reduce the manufacturing cost by making the rawmaterial shareable between the supporting members 317B and

Fourth Embodiment

A fourth embodiment of the present invention will be described withreference to FIG. 12. This embodiment has a feature such that, in thefuel spacer positioned above the upper ends of the first short-lengthfuel rods 2 b 1, the flow tabs are partially omitted. FIG. 12 is a topview showing the structure of a fuel spacer 404 b in this embodiment. Inthe fuel spacer 404 b, parts common to those in the fuel spacer 4 bdescribed in the first embodiment with reference to FIG. 1 aredesignated by the same symbols and an explanation thereof will beomitted.

The fuel spacer 404 b shown in FIG. 12 is different from the fuel spacer4 b shown in FIG. 1 in that, of the large number of the flow tabs 15formed on the band 11 for introducing the flow of a coolant, thoselocated at positions adjacent to the supporting member 17 for connectingthe cells 9A and 9B to the band 11 at each first lattice position 7 aassociated with the first short-length fuel rod 2 b 1 are omitted. Theremaining configuration is substantially the same as that in the firstembodiment.

This embodiment having the above configuration exhibits the followingeffects:

The flow tabs 15 provided on the band 11 of the fuel spacer 404 b have afunction of directing the flow of a coolant in the fuel assembly 1toward the fuel rod 2 side as much as possible, thereby improving thecooling effect of the fuel rods 2 and enhancing the critical powercharacteristic. The flow tabs 15, however, have an inconvenience inthat, since the projecting shapes of the flow tabs 15 obstruct the flowof the coolant, the pressure loss is correspondingly increased.

Incidentally, since the fuel spacer 404 b is positioned above the upperends of the short-length fuel rods 2 b 1, no fuel rods are present atthe lattice positions associated with the short-length fuel rods 2 b 1and instead the supporting members 17 are provided at the latticepositions. Accordingly, the provision of the flow tabs 15 in thevicinity of the lattice positions does not improve the critical powercharacteristic, but causes a problem in that it increases the pressureloss. For this reason, the flow tabs 15 adjacent to each supportingmember 17 may be omitted. This is effective to eliminate an increase inpressure loss due to the projecting shapes of the flow tabs 15, andhence to further reduce the pressure loss.

In the fourth embodiment, the lattice position associated with the firstshort-length fuel rod 2 b 1 is located at the intermediate position oneach of the four sides of the outermost peripheral region of the squarelattice array and the flow tabs 15 adjacent thereto are omitted;however, the present invention is not limited thereto. For example, thelattice position associated with the first short-length fuel rod 2 b 1may be located at any position in the outermost peripheral region,preferably, except for four corners of the square lattice array. Thereason for this will be described below.

The effect of improving the critical power characteristic due to theflow tabs 15 becomes largest at the four corners in the outermostperipheral region of the square lattice array and becomes smaller atother positions in the outermost peripheral region. Accordingly, in thecase where the lattice position associated with the first short-lengthfuel rod 2 b 1 is positioned in the outermost peripheral region exceptfor the four corners and the supporting member 17 is provided at thelattice position, the effect of improving the critical powercharacteristic is not reduced so much even if the flow tabs 15 adjacentto the supporting member 17 are omitted; while the effect of reducingthe pressure loss due to omission of the flow tabs 15 becomes largeirrespective of the arrangement of the flow tabs 15. As a result, it maybe desirable to locate the lattice position associated with the firstshort-length fuel rod 2 b 1 in the outermost peripheral region exceptfor the four corners and omit the flow tabs 15 adjacent to thesupporting member 17 located at the lattice position associated with thefirst short-length fuel rod 2 b 1.

In the fourth embodiment, only the flow tabs 15 adjacent to the latticeposition associated with the first short-length fuel rod 2 b 1 in theconfiguration of the first embodiment are omitted; however, the presentinvention is not limited thereto. For example, the flow tabs 15 at otherpositions may be omitted insofar as an effect exerted on the criticalpower characteristic due to omission of the flow tabs 15 is allowable.In this case, it is possible to further reduce the pressure loss.Further, the flow tabs 15 in each configuration of the second and thirdembodiments may be partially omitted.

In each of the first to fourth embodiments, the present invention isapplied to the fuel assembly in which the two water rods are arranged inthe region in which the seven fuel rods 2 are arrangeable; however, thepresent invention is not limited thereto, but can be applied to a fuelassembly in which one or three or more water rods are arranged in aregion in which six or less or eight or less of the fuel rods 2 arearrangeable.

The present invention can be also applied to a fuel assembly including asquare type water rod formed to have a square shape in transversecross-section. FIG. 13 shows a fuel spacer 4 bB of a fuel assemblyincluding such a square type water rod, wherein the fuel spacer 4 bB ismodified from the fuel spacer 4 b shown in FIG. 1. The fuel spacer 4 bBshown in FIG. 13 is different from the fuel spacer 4 b shown in FIG. 1in that a square type water rod holding member 12A is provided at thecentral portion of the fuel spacer 4 bB and the number of the cells 9 iscorrespondingly reduced to 68 cells. The remaining configuration issubstantially the same as that shown in FIG. 1. The fuel spacer 4 bBexhibits an effect similar to that obtained by the fuel spacer 4 b.

In each of the first to fourth embodiments, the present invention isapplied to a fuel assembly including second short-length fuel rods 2 b 2in addition to the first short-length fuel rods 2 b 1; however, thepresent invention is not limited thereto. For example, the presentinvention can be applied to a fuel assembly in which fuel rods eachhaving the normal fuel active length are located at the second latticepositions in place of the second short-length fuel rods 2 b 2 or no fuelrods may be provided at the second lattice positions. In this case, thesame effect can be obtained.

In each of the first to four embodiments, the first short-length fuelrods 2 b 1 are dispersedly located in the outermost peripheral region ofthe square lattice array, that is, two or more of the first short-lengthfuel rods 2 b 1 are located so as to be not adjacent to each other inthe outermost peripheral region, however, the present invention is notlimited thereto. For example, even in the case where two or more of theshort-length fuel rods are arranged in the outermost peripheral regionas disclosed in Japanese Patent Laid-open No. Hei 6-2373, the cells atthe lattice positions associated with the short-length fuel rods can beremoved and instead members similar to the supporting members 17, 217 or317 shown in the first to third embodiments can be provided at thelattice positions. Even in this case, there can be obtained a strengthensuring effect comparable to that obtained in each of theabove-described embodiments.

Fifth Embodiment

A fifth embodiment of the present invention will be described withreference to FIGS. 14 to 20. This embodiment has a feature such that thenecessary minimum number of one kind of the loop-shaped springs arereasonably arranged over an entire fuel spacer. In this embodiment,parts common to those in the first embodiment are designated by the samesymbols and explanation thereof will be omitted.

FIG. 14 is a top view showing the structure of a fuel spacer 504 b inthis embodiment. As shown in FIG. 14, the fuel spacer 504 b in thisembodiment is configured such that the cells 9 at the first latticepositions 7 a associated with the first short-length fuel rods 2 b 1 andthe second lattice positions 7 b associated with the second short-lengthfuel rods 2 b 2 are removed, and correspondingly, the arrangement of theloop-shaped springs 10 over the fuel spacer 504 b is entirely changedfrom that in the fuel spacer 4 a shown in FIG. 4. That is to say, withrespect to the cell 9C adjacent in the same row to one of the two secondlattice positions 7 b adjacent to the water rods 3 and the cell 9Dadjacent in the same column to the above second lattice position 7 b,each of the cells 9C and 9D includes, at a portion on the second latticeposition 7 b side, a known spring supporting portion. A loop-shapedspring 10A (10B) for pressing against the fuel rod 2 inserted in thecell 9C (9D) is provided in the above spring supporting portion providedin the cell 9C (9D).

The loop-shaped spring 10A (10B) is, as will be described later,supported by an approximately cylindrical spring pressing member 19provided at the second lattice position 7 b in such a manner as togenerate a pressing force applied to the fuel rod 2 inserted in the cell9C (9D). The spring pressing member 19 is made as thin as possiblewithin an allowable range in terms of the structural strength of thefuel spacer. That is to say, to reduce the pressure loss the transversecross-section of the spring pressing member 19 is made smaller than thetransverse cross-section of the cell 9. In addition, the spring pressingmember 19 can be manufactured commonly using the raw circular tube forforming the cell 9. This is effective to reduce the manufacturing costby making the raw material shareable between the spring pressing member19 and the cell 9. The water rod holding member 12 formed into theΩ-shape in transverse cross-section is joined to each of the two springpressing members 19.

The detailed structure near the two second lattice positions 7 b, whichforms the largest difference between the fuel spacer 504 b and the fuelspacer 4 a, will be described below.

(1) Fuel Spacer 4 a

FIG. 15 is a transverse sectional view showing the structure near thetwo second lattice positions 7 b of the fuel spacer 4 a; and FIG. 16 isa perspective view showing the structure near the joined portionsbetween the water rod holding member 12 and the cells 9. In addition,for convenience in description, the fuel rods 2 and the water rods 3 areadditionally shown in FIG. 15.

Referring to FIGS. 15 and 16, the water rod holding member 12 includestwo spring holding projecting pieces 12 a each of which projects in theshape of tongue in the loop of the loop-shaped spring 10 for holding theloop-shaped spring 10, and a spring pressing projecting piece 12 b whichprojects in the shape of tongue in such a manner as to be brought incontact with the loop of the loop-shaped spring 10 from the outerperipheral side.

The two spring holding projecting pieces 12 a and the spring pressingprojecting piece 12 b all project in the same direction (leftward inFIG. 16), and the two spring holding projecting pieces 12 a are providedabove and below the spring pressing projecting piece 12 b, respectively.These spring holding projecting pieces 12 a and the spring pressingprojecting piece 12 b are manufactured by press-working a base portionof the water rod holding member 12 to form tongue-shaped cut piecescorresponding to the projecting pieces 12 a and 12 b and two windows 12c. The spring pressing projecting piece 12 b is finished by bending thecorresponding tongue-shaped cut piece from its root and then flatteningit. The windows 12 c are provided to provide spaces in which theloop-shaped spring 10 is inserted when the loop-shaped spring 10 ismounted.

The cell 9 to be joined to the water rod holding member 12 has twowindows 9 b (only one is shown for simplicity) and a projecting piece 9c which projects in the direction opposed to the projecting direction ofthe projecting pieces 12 a. The two spring holding projecting pieces 12a are in contact with the projecting piece 9 c. The contact portions ofthe spring holding projecting pieces 12 a with the cell projecting piece9 c are inserted in the loop of the loop-shaped spring 10, so that thevertical movement of the looped spring 10 may be restricted. Inaddition, both ends of the windows 12 c and 9 b form removal preventiveportions 12 c 1 and 9 b 1 (only partially shown) for preventing theremoval of the loop-shaped spring 10.

The spring pressing projecting piece 12 b functions to restrict thehorizontal displacement of the loop-shaped spring 10 due toexpansion/contraction thereof, and hence to generate a pressing forceapplied to the fuel rod 2 inserted in the cell 9. At this time, as shownin FIG. 15, the distance d1 between the spring pressing projecting piece12 b and the adjacent fuel rod 2 is made equal to the distance d2between the two adjacent fuel rods 2. With this configuration, apressing force of the loop-shaped spring 10 generated when the spring 10is pressed by the spring pressing projecting piece 12 b is made equal toa spring pressing force of the loop-shaped spring 10 generated when thespring 10 is held between the two fuel rods 2. As a result, the fuel rod2 can be suitably fixed in the cell 9 at the lattice position 7 b.

(11) Fuel Spacer 504 b

FIG. 17 is a perspective view, with parts partially cutaway, showing thestructure near the joined portions between the spring pressing member 19and the cells 9 in the fuel spacer 504 b, which is an essential portionof this embodiment. Referring to FIG. 17, the spring pressing member 19includes two spring holding projecting pieces 19 a and two springholding projecting pieces 19 b, which function as spring holdingportions projecting in the shape of a tongue inserted in the loops ofthe loop-shaped springs 10A and 10B for holding the loop-shaped springs10A and 10B, respectively; and two spring pressing projecting pieces 19Cand 19D which function as spring pressing portions projecting in theshape of a tongue in such a manner as to be in contact with the loops ofthe loop-shaped springs 10A and 10B from the outer peripheral side,respectively.

The structures and functions of these projecting pieces 19 a (19 b) and19 c (19 d) are similar to those of the projecting pieces 12 a and 12 bof the water rod holding member 12 described in the paragraph (1),respectively. To be more specific, the two spring holding projectingpieces 19 a are provided above and below the spring pressing projectingpiece 19 c, respectively; and the two spring holding projecting pieces19 b are provided above and below the spring pressing projecting piece19 d, respectively. These spring holding projecting pieces 19 a and 19 band the spring pressing projecting pieces 19 c and 19 d are manufacturedby press-working a cylindrical base portion 19 g of the spring pressingmember 19 to form tongue-shaped cut pieces corresponding to theprojecting pieces 19 a, 19 b and 19 c and 19 d and four windows 19 e and19 f. Each of the spring pressing projecting pieces 19 c and 19 d isfinished by bending the corresponding tongue-shaped cut piece from itsroot and then flattening it.

At this time, the two spring holding pieces 19 a (19 b) are in contactwith a projecting piece 9Cc (9Dc) formed in the cell 9C (9D), and thecontact portions of the spring holding projecting pieces 19 a (19 b),with the cell projecting piece 9Cc (9Dc) are inserted in the loop-shapedspring 11A (10B). In addition, to prevent the removal of the loop-shapedspring 10A (10B), the ends of windows 19 e (19 f) of the loop-shapedspring 10A (10B) have removal preventive portions 19 e 1 (19 f 1) andthe ends of the windows of the cell 9C (9D) have removal preventiveportions (not shown for simplicity).

The spring pressing projecting piece 19 c (19 d) functions to restrictthe horizontal displacement of the loop-shaped spring 10A (10B) due toexpansion/contraction thereof, and hence to generate a suitable pressingforce (similar to the pressing force described in the paragraph (1)) tothe fuel rod 2 inserted in the cell 9C (9D).

Here, there is a large structural difference between the projectingpieces 19 a, 19 b, 19 c and 19 d and the projecting pieces 12 a and 12 bof the water rod holding member 12 described in the paragraph (1) inthat the spring holding projecting pieces 19 a and 19 b all project inthe same direction (leftward in FIG. 17) and one spring pressingprojecting piece 19 d also projects in the same direction; however, theother spring pressing projecting piece 19 c projects in the oppositedirection (rightward in FIG. 17).

The function of this embodiment having the above configuration will bedescribed below.

(1) Improvement in Reactivity Controllability In the fuel assembly 1 inthis embodiment, since six of the short-length fuel rods 2 b areincluded with the fuel rods 2 arranged in the square lattice array of 9rows×9 columns, it is possible to equalize the H/U ratio by making useof a saturated water region formed on the upper side of the short-lengthfuel rods 2 b. At this time, by arranging the short-length fuel rods 2 bat the lattice positions in the outermost peripheral region of thesquare lattice array and at the lattice positions adjacent to the waterrods, it is possible to more effectively improve the controllability ofthe reactivity by reducing the void coefficient as disclosed in JapanesePatent Laid-open No. Hei 5-232273.

(2) Reduction in Pressure Loss

Since the unnecessary cells 9 at the first and second lattice positions7 a and 7 b in the fuel spacer 4 b positioned above the upper ends ofthe short-length fuel rods 2 b are omitted, the pressure loss can becorrespondingly reduced. In addition, a spring pressing member 19 isprovided in place of the cell 9 at each second lattice position 7 b;however, since the transverse cross-section of the spring pressingmember 19 is made smaller than that of the cell 9 as described above, itis possible to reduce the pressure loss. 1

(3) Reasonable Arrangement of Spring

As a result of removing the cells 9 in the fuel spacer 504 b forreducing the pressure loss (see the paragraph (2)), the arrangement ofthe loop-shaped springs 10 over the entire fuel spacer is necessarilychanged such that the loop-shaped springs 10A and 10B are respectivelyprovided on portions, on the second lattice position 7 b side, of thecells 9C and 9D located at the lattice positions adjacent to each of thetwo second lattice positions 7 b for pressing against the fuel rods 2 inthe cells 9C and 9D. The usual loop-shaped spring 10 functions togenerate pressing forces when the fuel rods 2 are inserted in a pair ofthe adjacent cells 9, and accordingly, if such usual loop-shaped springsare used for the loop-shaped springs 10A and 10B, the loop-shapedsprings 10A and 10B are made free on the second lattice position 7 bside and thereby cannot generate the pressing forces.

Incidentally, the structure in which one loopshaped spring which is freeon one side is supported at one lattice position in such a manner as togenerate a suitable pressing force has been known, for example, asrepresented by the structure shown in FIG. 16 or the structure disclosedin Japanese Patent Laid-open No. Hei 6-273560; however, theabove-described structure in which the two loop-shaped springs 10A and10B each being free on one side are supported at one lattice positionhas not been known.

On the contrary, in the fuel spacer 504 b in this embodiment, since thespring pressing member 19, which includes the four spring holdingprojecting pieces 19 a and 19 b and the two spring pressing projectingpieces 19 c and 19 d, is provided at one of two second lattice positions7 b, it is possible to support the loop-shaped springs 10A and 10B sothat each is free on one side and hence to generate suitable pressingforces. This makes it possible to reasonably arrange the necessaryminimum number (36 pieces) of the loop-shaped springs 10 over the entirefuel spacer without increasing the kinds of springs being used, morespecifically, using only one kind of the springs.

(4) Attainment of Rigidity/Strength of Spring Pressing Member toPressing Force of Spring

This function will be described with reference to a comparative example.FIG. 18 is a perspective view, with parts partially cutaway, showing thestructure near the joined portions between a spring pressing member andthe cells in the comparative example. In FIG. 18, parts common to thosein FIG. 17 are designated by the same symbols.

In the comparative example shown in FIG. 18, two of the structures shownin FIG. 16, each being similar to that disclosed in Japanese PatentLaid-open No. Hei 6-273560, are simply arranged for supporting the twoloop-shaped springs 10A and 10B so that each is free on one side at onesecond lattice position 7 b. The structure shown in FIG. 18 is differentfrom that shown in FIG. 17 in that the spring holding projecting pieces19 a and 19 b all project in the same direction (leftward in FIG. 18)and the two spring pressing projecting pieces 19 c and 19 d project inthe same direction (leftward in FIG. 18).

Such a structure as shown in FIG. 18 has the following inconvenience.That is to say, to bring the spring pressing projecting pieces 19 c and19 d in contact with the loops of the loop-shaped springs 10A and 10Bfrom the outer peripheral side, the spring pressing projecting pieces 19c and 19 d are required to project on the inner side (toward the frontin FIG. 18) of the spring pressing member 19 more than the springholding projecting pieces 19 a and 19 b inserted in the loops.

Accordingly, if the projecting direction of all of the spring pressingprojecting pieces 19 c and 19 d is made identical to the projectingdirection of the spring holding projecting pieces 19 a and 19 b, thesize of the cutouts on both sides of the spring pressing projectingpieces 19 c and 19 d must be made larger, and correspondingly the width(or area) of a bridge 19 g 1, equivalent to the root portion of thespring pressing projecting piece 19 c, of the base plate portion 19 g ofthe spring pressing member 19 becomes smaller. This makes it difficultto ensure a sufficient strength and rigidity against the pressing forceof the loop-shaped spring 10A.

On the contrary, according to the configuration of this embodiment asshown in FIG. 17, since the projecting direction of one spring pressingprojecting piece 19 c is reversed relative to the projecting directionof the spring holding projecting pieces 19 a and 19 b, the width (orarea) of the bridge 19 g 1 can be made larger. This makes it possible toensure a sufficient strength and rigidity.

As described above, according to this embodiment, the fuel assembly 1 isconfigured such that the shortlength fuel rods 2 b are arranged in theoutermost peripheral region of the square lattice array of 9 rows×9columns adjacent to the water rods 3, and at the fuel spacer 4 b, thecells 9 at the lattice positions 7 a and 7 b associated with theshort-length fuel rods 2 b are removed to reduce the pressure loss. Inthis fuel assembly 1, the two loop-shaped springs 10A and 10B, eachbeing free on one side, are supported by the spring pressing member 19at one second lattice position 7 b, so that the necessary minimum numberof the loop-shaped springs 10 may be reasonably arranged over the entirefuel spacer without increasing kinds of the springs being used.

Since the number of the loop-shaped springs 10 is selected at thenecessary minimum value, there can be obtained an effect of furtherreducing the pressure loss, and since the width (or area) of the bridge19 g 1 can be made larger, there can be obtained an effect of ensuring asufficient strength and rigidity against the pressing forces of theloop-shaped springs 10A and 10B.

In the fifth embodiment, the spring holding projecting pieces 19 a and19 b all project in the same direction (leftward in FIG. 17) and onespring pressing projecting piece 19 d also projects in the samedirection; while the other spring pressing projecting piece 19 cprojects in the opposite direction (rightward in FIG. 17); however, thestructure of the projecting pieces is not limited thereto.

Hereinafter, two modifications will be described with reference to FIGS.19 and 20. FIG. 19 is a perspective view, with parts partially cutaway,showing the structure near the joined portions between the springpressing member and the cells according to the first modification. InFIG. 19, parts common to those in FIG. 17 are designated by the samesymbols. The structure shown in FIG. 19 is different from that shown inFIG. 17 in that the spring holding projecting pieces 19 a and 19 b allproject in the same direction (leftward in FIG. 19), while both springpressing projecting pieces 19 c and 19 d project in the oppositedirection (rightward in FIG. 19).

With this structure, since the width (or area) of the bridge 19 g 1 canbe made larger than that in the comparative example shown in FIG. 18, itis possible to ensure a sufficient strength and rigidity against thepressing forces of the loop-shaped springs 10A and 10B. That is to say,it becomes apparent that at least one of the spring pressing projectingpieces may project in the direction opposite to the projecting directionof the spring holding projecting pieces.

FIG. 20 is a perspective view, with parts partially cutaway, showing thestructure near the joined portions between the spring pressing memberand the cells according to the second modification. In FIG. 20, partscommon to those in FIG. 17 are designated by the same symbols. Thestructure shown in FIG. 20 is different from that shown in FIG. 17 inthat the opposed free ends of the spring pressing projecting pieces 19 cand 19 d in the structure shown in FIG. 17 are connected to the baseplate portion 19 g of the spring pressing member 19, that is, as shownin FIG. 19, both right and left ends of each of the projecting pieces 19c and 19 d are connected to the base plate portion 19 g so as to beintegrally formed therewith.

In the second modification, when the pressing forces of the loop-shapedsprings 10A and 10B are applied to the spring pressing projecting pieces19 c and 19 d, they can be supported by the base plate portion 19 gconnected to both sides of each of the spring pressing projecting pieces19 c and 19 d. This is effective to ensure a sufficient strength andrigidity.

The second modification has another effect. In the structures shown inFIGS. 17 and 19, as described above, the spring pressing projectingpieces 19 c and 19 d are manufactured by forming tongue-shaped cutpieces in the base plate portion 19 g of the spring pressing member 19,bending the cut pieces from the roots thereof and flattening them. Incontrast, in this modification, the spring pressing projecting pieces 19c and 19 d can be simply manufactured merely by forming cut linescorresponding to both side lines of the projecting pieces 19 c and 19 din the base plate portion 19 g of the spring pressing member 19 andpressing the portions surrounded by the cut lines such that the portionsproject inward of the spring pressing member 19. This is effective toreduce the manufacturing cost.

While the spring pressing member 19 is formed into an approximatelycylindrical shape in the fifth embodiment, the present invention is notlimited thereto. For example, like the supporting member 218 in thesecond embodiment, the spring pressing member 19 may be formed into anapproximately polygonal shape from which a partial side portion isremoved for reducing the pressure loss. As shown in FIG. 21, flow tabs17 having the same function as that of the flow tabs 15 provided on theband 11 may be provided on the spring pressing member 19. The flow tabs17 project outward from the outer periphery of the spring pressingmember 19 for introducing the flow of a coolant in the projectingdirection of the flow tab 17. The structure including the flow tabs 17exhibits the following effect.

The spring pressing member 19 at the second lattice position 7 b, whichis at the level in which no fuel rod 2 is present, is not required to becooled. Accordingly, to direct the flow of a coolant passing through thespring pressing member 19 toward the other fuel rods 2 around thelattice position 7 b as much as possible, the flow tabs 17 are providedon the spring pressing member 19. This makes effective use of thecoolant and hence improves the effect of cooling the fuel rods 2.

Sixth Embodiment

A sixth embodiment of the present invention will be described withreference to FIG. 22. In this embodiment, a spring pressing memberhaving a shape different from that in the fuel spacer 504 b of the fifthembodiment is provided. Parts common to those in FIG. 1 are designatedby the same symbols and an explanation thereof will be omitted. FIG. 22is a top view showing the structure of a fuel spacer 604 b in thisembodiment. In addition, the water rods 3 are also shown in the figurefor more clearly showing the structure of the fuel spacer 604 b.

Like the fuel spacer 504 b, the fuel spacer 604 b shown in FIG. 22 ispositioned above the upper ends of the short-length fuel rods 2 b in thefuel assembly 1 shown in FIG. 2 or FIG. 3. In the fuel spacer 604 b, thespring pressing member 19 of the fuel spacer 504 b shown in FIG. 14 isreplaced with a spring pressing member 619, formed into an umbrellashape in transverse cross-section, which has a function of the springpressing member 19 in combination with the function of the water rodholding member 12.

The structure of the spring pressing member 619 will be brieflydescribed. Like the spring pressing member 19 shown in FIG. 14, thespring pressing member 619 includes spring holding projecting pieces(not shown) which are inserted in the loops of the loop-shaped springs10A and 10B for holding the loop-shaped springs 10A and 10B; and springpressing projecting pieces 619 a and 619 b which are brought in contactwith the loops of the loop-shaped springs 10A and 10B from the outerperipheral side. The spring pressing member 619 also includes a waterrod holding portion 619 c which functions to hold the water rod 3 in theradial and axial directions, like, the water rod holding member 12 shownin FIG. 14. The remaining configuration is substantially the same asthat of the fuel spacer 504 b in the fifth embodiment.

The fuel assembly including the fuel spacer 604 b in this embodimentexhibits an effect comparable to that of the fuel assembly including thefuel spacer 504 b in the fifth embodiment. Additionally, according tothis embodiment, since the spring pressing member 19 and the water rodholding member 12 are replaced with the spring pressing member 619, itis possible to reduce the number of parts, and hence to lower themanufacturing cost; and also it is possible to reduce the transversecross section of the portion adjacent to the water rods 3, and hence tofurther reduce the pressure loss.

In the sixth embodiment, the structures of the spring pressingprojecting pieces 619 a and 619 b and the spring pressing projectingpieces are not limited to those shown in FIG. 17, but may be similar tothose shown in FIG. 19 or 20.

In the fifth and sixth embodiments, description is made by way ofexample of a fuel assembly in which the fuel rods 2 are located in asquare lattice array of 9 rows×9 columns; however, the present inventioncan be applied to a fuel assembly having a square lattice array of 8rows×8 columns or 10 rows×10 columns, or a rectangular lattice array inwhich the rows and columns are different in number. Even in such a fuelassembly, if two of the loop-shaped springs are required to be supportedat one lattice position, the concept of the present invention can beapplied thereto, with a result that the same effect can be obtained.

In a fuel assembly having a square lattice array of 9 rows×9 columns inwhich the arrangement of the loopshaped springs 10 is different fromthat shown in FIG. 14 or FIG. 22, if two of the loop-shaped springs arerequired to be supported at one lattice position, the concept of thepresent invention can be applied thereto, with a result that the sameeffect can be obtained.

Even in the case where three or more of the loop-shaped springs 10 arerequired to be supported at one lattice position, the concept of thepresent invention can be applied thereto. Such a modification will bedescribed with reference to FIG. 23. FIG. 23 is a top view showing thestructure of an essential portion of a fuel spacer 704 b in thismodification. Like the fuel spacers 504 b and 604 b, the fuel spacer 704b shown in FIG. 23 is positioned above the upper ends of theshort-length fuel rods 2 in the fuel assembly 1 shown in FIG. 2 or 3.

In the case of reviewing the arrangement of the loopshaped springs 10 inaccordance with removal of the cells 9 to reduce the pressure loss,there may occur a requirement in which four free loop-shaped springs710A to 710D (each having the same structure as that of the loop-shapedspring 10) need to be supported at one lattice position, other thanthose in the outermost peripheral region of the square lattice array andthose adjacent to the water rods 3. To meet such a requirement, the fuelspacer 704 b is provided with a cylindrical spring pressing member 719for supporting the springs 710A to 710D in such a manner that thesprings can generate suitable pressing forces.

The structure of the spring pressing member 719 will be brieflydescribed. Like the spring pressing member 19 shown in FIG. 19, thespring pressing member 719 includes four spring holding projectingpieces (not shown) which are inserted in the loops of the fourloop-shaped springs 710A to 710D surrounding the spring pressing member719 for holding the loop-shaped springs 710A to 710D; and four springpressing projecting pieces 719 a to 719 d which are brought in contactwith the loops of the loop-shaped springs 710A to 710D from the outerperipheral side. The spring pressing member 719 is also made as thin aspossible within an allowable thickness range in terms of the structuralstrength of the fuel spacer for making the transverse cross-sectionthereof smaller than that of the cell 9, thereby reducing the pressureloss. The remaining configuration is substantially the same as that ofthe fuel spacer 504 b in the fifth embodiment.

Like the fifth embodiment, the fuel assembly including the fuel spacer704 b in this embodiment exhibits an effect of reasonably arranging thenecessary minimum number of the loop-shaped springs 710 over the fuelspacer without increasing the number of kinds of springs by supportingthe four free loop-shaped springs 710A to 710D at one lattice positionby means of the spring pressing member 719.

In the above modification, description is made by way of example of thecase of providing four free loop-shaped springs 710A to 710D; however,the present invention is not limited thereto, but may be applied to thecase of providing only three of the free loop-shaped springs. In thiscase, by providing three sets of the spring pressing projecting pieces719 a to 719 c and the spring holding projecting pieces, an effectsimilar to that described above can be obtained.

Since the variations in allowable arrangement of the loop-shaped springscan be further increased by making the spring pressing member 719 in theabove modification in combination with the spring pressing members 19and 619 in the fifth and sixth embodiments, this arrangement is expectedto provide a more effective fuel spacer from the viewpoint of reductionin pressure loss.

In each of the first to sixth embodiments, description is made by way ofexample of a fuel assembly in which the fuel rods 2 are located in asquare lattice array of 9 rows×9 columns; however, the present inventionis not limited thereto, but may be applied to another fuel assemblyhaving a square lattice array of 8 rows×8 columns, 10 rows×10 columns,or the like. In this case, an effect similar to that described above canbe obtained.

While the preferred embodiments of the present invention have beendescribed using specific examples, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the followingclaims.

What is claimed is:
 1. A fuel assembly comprising: a plurality of fuelrods arranged in a square lattice array, said fuel rods including aplurality of short-length fuel rods each having a fuel active lengthshorter than that of each of the remaining ones of said fuel rods; atleast one water rod arranged in a region in which one or more of saidfuel rods are arrangeable in said array; and a plurality of fuelspacers, provided at a plurality of positions in the axial direction,for holding said plurality of fuel rods and said at least one water rodwith mutual radial intervals therebetween kept immovable; wherein saidplurality of short-length fuel rods include at least one firstshort-length fuel rod arranged in the outermost peripheral region ofsaid square lattice array; each of said plurality of fuel spacersincluding a plurality of cylindrical members which are connected to eachother and in which said fuel rods are to be inserted respectively, and aband member for surrounding the outermost peripheries of said pluralityof cylindrical members; and said plurality of fuel spacers includingfirst-fuel spacers positioned above the upper end of said at least onefirst short-length fuel rod, and at least one of said first fuel spacersis configured such that one of said cylindrical members, located at afirst lattice position associated with said at least one firstshort-length fuel rod, is omitted, and, instead, a first supportingmember is provided at said first lattice position for directlyconnecting said band member to two first cylindrical members of saidplurality of cylindrical members which are adjacently located on bothsides of said first lattice position in the outermost peripheral regionadjacent to said band member.
 2. A fuel assembly according to claim 1,wherein said first supporting member has a transverse cross-sectionsmaller than that of a cylindrical member.
 3. A fuel assembly accordingto claim 1, wherein said at least one first fuel spacer includes, at aconnection portion between one of said two, first cylindrical membersand said first supporting member, spring pressing means for imparting apressing force to a spring for holding said fuel rod inserted in saidone of said two first cylindrical members.
 4. A fuel assembly accordingto claim 1, wherein said plurality of short-length fuel rods include atleast one second short-length fuel rod arranged in a region adjacent tosaid at least one water rod; each of said plurality of fuel spacersincludes a water rod holding member connected to those of said pluralityof cylindrical members which are arranged in the innermost peripheralregion of said square lattice array for holding said at least one waterrod; and said at least one first fuel spacer is configured such that oneof said cylindrical members, located at a second lattice positionassociated with said at least one second short-length fuel rod, isomitted, and, instead, another supporting member for connecting twoother cylindrical members, of said plurality of cylindrical members,located outwardly from and adjacently to said second lattice position insaid square lattice array, to said water rod holding member, is providedat said first lattice position.
 5. A fuel assembly according to claim 1,wherein each of said plurality of fuel spacers includes a plurality offirst projecting members provided on said band member, said firstprojecting member projecting between two adjacent cylindrical members inthe outermost peripheral region of said square lattice array forintroducing the flow of a coolant; and said at least one first fuelspacer is configured such that at least one of said projecting membersadjacent to said first supporting member is omitted.
 6. A fuel assemblyaccording to claim 5, wherein said at least one first short-length fuelrod is arranged in the outermost peripheral region except for fourcorners of said square lattice array.